1. Field of the Invention
The present invention relates to an optical pickup for recording or reproducing information on or from information recording media such as CD and DVD by irradiating a light beam onto the information recording media.
2. Description of the Related Art
Conventionally, an optical pickup to be used for reproduction and the like from an information recording medium such as DVD is constituted so that a main-beam is irradiated in order to detect information pits on a recording track and a plurality of sub-beams are irradiated onto predetermined positions for various objects such as a tracking information detection by means of the a so-called three-beam method and an operation of a cross talk canceller in order to suppress cross talk from adjacent tracks. For this reason, it is generally performed that a light beam is diffracted by a diffraction grating to produce diffracted lights which are irradiated as sub-beams.
Recently, in order to realize the diffraction grating for use in an optical pickup, a liquid crystal panel has been utilized. Namely, a split form of the liquid crystal panel is formed as a grating pattern, and the diffracted light can be irradiated as sub-beams by giving a phase difference due to a refractive index change to the light beam passing through the liquid crystal panel according to a birefringence effect of the liquid crystal.
In such a liquid crystal panel, a desired diffraction efficiency can be obtained by setting the split form suitably, and the phase change to be given to the light beams can be adjusted by controlling an applied voltage to the liquid crystal panel. Moreover, it is comparatively easy to perform switched control between the irradiation of only a main-beam irradiation and three beams or the like. Therefore, when the liquid crystal panel is used as the diffraction grating, it is possible to provide an optical pickup which is effective for recording on or reproduction from DVD and the like.
However, it is not easy to operate the conventional optical pickup utilizing the liquid crystal panel with changing the irradiated positions of the main-beam and sub-beams. Namely, the distances of beam spots of the main-beam and sub-beams change in inverse proportion to a grating pitch of the diffraction gratings. While the phase difference can be changed by controlling the applied voltage to the liquid crystal panel, the distances of the beam spots should be fixed because the grating pitch is preset as the split form.
In addition, it is conceivable to provide and suitably switch two kinds of liquid crystal panels on which the diffraction grating are formed with different pitches. However, if the optical pickup is constituted in such a manner, it is not easy to adjust the orientation of the liquid crystals relative to the light beams and the positioning of the liquid crystal panels, and thus this causes a rise in the cost.